A phase change material is a material which undergoes a phase change, typically between the liquid and solid phases. Phase change materials are frequently used in energy storage applications because larger amounts of energy can be stored as latent heat, i.e. the energy released by solidification or required for liquefaction, than as sensible heat, i.e. the energy needed to increase the temperature of a single phase material.
One phase change material that has come into widespread use is a salt of magnesium nitrate hexahydrate and lithium nitrate. Several different formulations have been proposed for the ratio of magnesium nitrate hexahydrate to lithium nitrate used to prepare the salt. EPO Patent No. 365,623 discloses a salt formed with magnesium nitrate hexahydrate/lithium nitrate ratios of between 92:8 and 87:13. EPO Patent No. 616,630 discloses a salt with ratios of between 86:14 and 81:19. The salts disclosed in the EPO Patents have a melting point temperature of about 70.degree. Centigrade and a latent heat on the order of 180 J/g. Additionally, the salts are also biodegradable and non-toxic.
These salts are particularly significant because the operating characteristics of the salts allow them to be used in heat batteries for energy storage in vehicles, such as automobiles. Their melting temperature is within a range that is suitable for efficient heat transfer in automotive applications, and the energy capacity of the material is large enough so that a significant amount of energy can be stored in a device of limited size and weight. A heat battery using the magnesium nitrate hexahydrate/lithium nitrate salts discussed above can release 750-800 Wh for a rapid discharge of 300 seconds.
However, the salts also have several disadvantageous characteristics which complicate their use in automotive heat batteries. Chiefly, the density of the salt in the solid state is much greater than the density of the salt in the liquid state. As a result, the density change during thermal cycling, which is reflected in a corresponding volume change, can cause deformation of the heat battery components as the material result, the density change during thermal cycling, which is reflected in a corresponding volume change, can cause deformation of the heat battery components as the material changes between the solid and liquid phases. The damage caused thereby can be exacerbated by localized melting of the salt, wherein the portion of the salt closest to the heat source melts first, forming a less dense liquid phase which forces the more dense solid phase against the components of the heat battery. In fact, it is possible for the change in density to deform the heat battery components to such an extent that the heat battery fails.
Even non-catastrophic cyclical deformation of the heat battery components can have farther ranging consequences when, as is normally the case, the heat battery is made of aluminum. Typically, aluminum will form an oxide layer which acts as a passivation layer or barrier to further corrosion of the material. The cyclic deformation of the heat battery components caused by the phase change material as the material transforms between liquid and solid phases causes defects to form in the protective aluminum oxide layer, thereby disrupting the protective passivation layer and opening the door to possible corrosion. At the defects, bare aluminum is exposed to nitrate compounds and corrodes. Considerable amounts of gases are generated within the battery as a consequence of the corrosion process. Eventually, the pressure from the increasing amount of corrosion gases contained within the heat battery causes the heat battery to rupture and fail.
One solution that has been proposed to limit the pressure buildup within the heat battery is to equip the heat battery with a relief valve to exhaust the corrosion gasses. This solution, however, adds significantly to the cost and complexity of the heat battery.